MIMO (multiple-input multiple-output) refers to a method for improving transmission/reception efficiency by adopting multiple transmit (Tx) antennas and multiple receive (Rx) antennas instead of a single Tx antenna and a single Rx antenna. That is, MIMO is a technology for increasing capacity or improving performance by using multiple antennas at a transmitting end or a receiving end of a wireless communication system. MIMO may be referred to as multi-antenna technology. To correctly perform multi-antenna transmission, it is necessary to feed back information on a channel from a receiving end that receives multiple antenna channels.
A rank indicator (RI), a precoding matrix index (PMI), a channel quality indicator (CQI), etc. are defined as information fed back from a receiving end to a transmitting end in a conventional multi-antenna wireless communication system. The RI, PMI and CQI are commonly referred to as channel state information (CSI). The CSI is configured to be suitable for multi-antenna transmission. In general, the RI may be determined as a value indicating a most suitable rank in the current channel state, the PMI may be determined as a value indicating a most suitable precoding matrix based on the rank and the CQI may be determined as a value indicating a most suitable modulation and coding scheme (MCS) based on the RI and PMI.